The present invention is directed to the field of annealers for processing sheet metal. An annealing step is typically performed in the manufacture of sheet metal, especially steel. Coils of metal are program heated up to a desired temperature and cooled down in order to relieve stresses within the metal.
Annealing systems typically include a furnace cover for establishing a containment perimeter for the furnace. A plurality of burners are retained within the wall of the furnace cover for firing horizontally inward toward the load. A stack of metal coils are retained inside an inner cover within the furnace. The inner cover encloses a chemically inert atmosphere around the load (typically 90% nitrogen and 10% hydrogen). This atmosphere serves as both the vehicle for transferring heat from the inner cover to the load as well as the protective atmosphere to prevent oxidation of the strip.
In previous systems, the best annealing quality is obtained using a single-stack annealer, i.e. one furnace and cover for each stack of coils. This type of system insures a high degree of temperature uniformity and control over numerous system variables. However, single-stack annealers are expensive to purchase since furnace covers are costly regardless of the size. However, single-stack annealers are capital intensive, primarily due o the cost of the furnace covers and base system, regardless of the size. Additionally, single-stack annealers require significantly more floor space per unit of production than do multi-stack furnaces.
These cost-effectiveness issues can be addressed by providing a multi-stack annealing furnace in which a plurality of stacks (typically between two and eight) are annealed within one furnace cover. Each stack has its own dedicated inner cover base fan, atmosphere supply and control thermocouple. All of the stacks are retained under a single furnace cover, which is a refractory-lined combustion chamber to which is mounted the burners and ancillary equipment. Such a furnace typically includes two thermocouples--one to limit the furnace temperature and the other as a master temperature control element. Due to capitalization costs, ambient air temperature and complexity, the furnace combustion system is designed to function as a single zone of control.
The furnace covers are raised, lowered and transported using a crane. During crane movement and positioning, it is not uncommon for the cover-mounted burners to be sheared off or damaged if the cover is dropped or misaligned during transporting and positioning of the cover. The inadvertent dropping of a coil exposes the furnace cover to similar damage.
A typical multi-stack annealing furnace uses flat flame burners alone or in combination with forward flame burners. Flat flame burners are designed to provide heat to the adjacent refractory in order to radiate heat back to the load. However, most modern annealing furnace designs have replaced traditional heavy refractory firebrick with lightweight ceramic fiber blankets. Such fiber blankets do not store heat, and thus cannot radiate heat to the load, thereby reducing heat transfer and efficiency of the system.
Forward flame burners are somewhat more effective at directly transferring heat to the load. However, forward flame burners produce a flame which can directly impinge upon the inner cover. Over time, the inner cover is burned up and destroyed by such impingement. Given that the cost of each furnace cover can currently exceed $15,000.00, such flame impingement significantly contributes to costs by shortening the useful life of the inner cover. Impingement can be reduced by enlarging the size of the furnace cover or reducing the outside diameter of the inner cover, which in turn dictates a reduction in the size of the load. However, this is also undesirable since the useful volume for retaining product is reduced, thus lowering yield for a given quantity of expended fuel.
Multi-stack annealers are less expensive to operate than single-stack systems due to such things as fewer crane lifts and costs associated with reduced floor space. Various factors influence the heating cycle of the coils within each stack, such as the weight and dimensions of the load and grade of material. Thermocouples can be used within each inner cover to monitor temperature for each stack. To prevent over heating of a particular stack, it is common for operators to manually adjust the burners, which can result in an upset of the air/fuel ratio, which in turn may provide for local impingement on an inner cover. Manual adjustments can also cause heating cycle delays, which results in inefficient fuel consumption and produce less yield per unit time. The multi-stack process has inherently always been 11/2 to 2 times longer than the single-stack process, and such delays contribute further to reduced yield and efficiency.
Manual adjustment also upsets the temperature balance in the system, creating thermal differentials throughout the load that upset the uniformity of the annealing process, resulting in lower-quality product as compared with the quality of the single-stack process. Flame supervision can be provided, but such usually assumes simultaneous burner operation. If any burner fails, the entire system shuts down, resulting in a significant loss of process time while spoiling the quality of the load. High ambient conditions generally reduce the life of the flame supervisory equipment.